DIMORPHISM OF FERNS. 345 



tions of the same. This is due to the fact that the tip of the 

 leaf develops later than the basal portions. At the time the 

 stimulus to the change in the development of the fertile leaves 

 reached them they were partly formed, that is the basal parts of 

 the fertile leaves were more or less developed and fixed and 

 could not change. Those portions of the leaf, however, which 

 were not yet completely formed, under this stimulus, or through 

 correlation of growth, are incited to vegetative growth, and ex- 

 pand more or less completely into vegetative leaves. 



628. The sporangia decrease as the fertile leaf expands. — 

 If we now examine the sporangia on the successive pinnae of a 

 partly transformed leaf we find that in case the lower pinna; are 

 not changed at all, the sporangia are normal. But as we pass to 

 the pinna? which show increasing changes, that is those which are 

 more and more expanded, we see that the number of sporangia 

 decrease, and many of them are sterile, that is they bear no 

 spores. Farther up there are only rudiments of sporangia, until 

 on the more expanded pinnae sporangia are no longer formed, 

 but one may still see traces of the indusium. On some of the 

 changed leaves the only evidences that the leaf began once to 

 form a fertile leaf are the traces of these indusia. In some of 

 these cases the transformed leaf was even larger than the sterile 

 leaf. 



629. The ostrich fern. — Similar changes were also produced 

 in the case of the ostrich fern, and in fig. 448 is shown at the 

 left a normal fertile leaf, then one partly changed, and at the 

 right one completely transformed. 



630. Dimorphism in tropical ferns. — Very interesting forms 

 of dimorphism are seen in some of the tropical ferns. One of 

 these is often seen growing in plant conservatories, and is known 

 as the staghorn fern (Platycerium alcicorne). This in nature 

 grows attached to the trunks of quite large trees at considerable 

 elevations on the tree, sometimes surrounding the tree with a 

 massive growth. One kind of leaf, which may be either fertile 

 or sterile, is narrow, and branched in a peculiar manner, so that 

 it resembles somewhat the branching of the horn of a stag 



